Can I drink water before a body-fat analysis? I’ve heard it might affect my results.

Imbibing pre-testing can skew scores. That’s because body-fat percentage is calculated by determining body composition, typically using a bioelectrical impedance analysis. A BIA machine (found in gyms and sold for home use) sends a safe low-voltage electric current through the body. Fatty tissue, which is about 10 to 20 percent water, slows the signal. On the flip side, it soars through fat-free mass, such as muscle, which contains 70 to 75 percent water. Flood your body and the device will give you a falsely low fat reading.

To get the best results, don’t exercise (it’s dehydrating) or sip two hours prior to your analysis, says F. Xavier Pi-Sunyer, M.D., director of the Obesity Research Center at St. Luke’s–Roosevelt Hospital Center in New York City. And even though an at-home test such as the Tanita BC-533 InnerScan Body Composition Monitor ($120; TheCompetitiveEdge.com) is an easy way to measure body fat and other stats, Dr. Pi-Sunyer explains it’s better to pinpoint your body-fat percentage (optimally 20 to 30 percent for women ages 20 to 40) at a medical center with underwater weighing. Performed correctly, results are accurate within 3 percent.